Starch consists of a mixture of amylose (15-30% w/w) and amylopectin (70-85% w/w). Amylose consists of linear chains of α-1,4-linked glucose units having a molecular weight (MW) from about 60,000 to about 800,000. Amylopectin is a branched polymer containing α-1,6 branch points every 24-30 glucose units; its MW may be as high as 100 million.
Sugars from starch, in the form of concentrated dextrose syrups, are currently produced by an enzyme catalyzed process involving: (1) liquefaction (or viscosity reduction) of solid starch with an α-amylase into dextrins having an average degree of polymerization of about 7-10, and (2) saccharification of the resulting liquefied starch (i.e. starch hydrolysate) with glucoamylase (also called amyloglucosidase or GA). The resulting syrup has a high glucose content. Much of the glucose syrup that is commercially produced is subsequently enzymatically isomerized to a dextrose/fructose mixture known as isosyrup. The resulting syrup also may be fermented with microorganisms, such as yeast, to produce commercial products including ethanol, citric acid, lactic acid, succinic acid, itaconic acid, monosodium glutamate, gluconates, lysine, other organic acids, other amino acids, and other biochemicals, for example. Fermentation and saccharification can be conducted simultaneously (i.e., an SSF process) to achieve greater economy and efficiency.
Glucoamylases (glucan 1,4-α-glucohydrolases, EC 3.2.1.3) are starch hydrolyzing exo-acting carbohydrases, which catalyze the removal of successive glucose units from the non-reducing ends of starch or related oligo and polysaccharide molecules. Glucoamylases can hydrolyze both the linear and branched glucosidic linkages of starch (e.g., amylose and amylopectin). α-Amylases, on the other hand, hydrolyze starch, glycogen, and related polysaccharides by cleaving internal α-1,4-glucosidic bonds at random. Glucoamylases have been used for a variety of different purposes, including starch saccharification, brewing, baking, production of syrups for the food industry, production of feedstocks for fermentation processes, and in animal feed to increase digestability.
Glucoamylases are produced by numerous strains of bacteria, fungi, and plants. For example, a glucoamylase is produced by strains of Aspergillus fumigatus. Luo et al. (2008) “Production of acid proof raw starch-digesting glucoamylase from a newly isolated strain of Aspergillus fumigatus MS-09,” Sci. Tech. Food Indus. 29(5): 151-154; Sellars et al. (1976) “Degradation of barley by Aspergillus fumigatus Fres,” Proc. Int. Biodegradation Symp., 3rd, S. J. Miles et al., eds., Appl. Sci., Barking, UK, pp. 635-43; Domingues et al. (1993) “Production of amylase by soil fungi and partial biochemical characterization of amylase of a selected strain (Aspergillus fumigatus Fresenius),” Can. J. Microbiol. 39(7): 681-85; Cherry et al. (2004) “Extracellular glucoamylase from the isolate Aspergillus fumigatus,” Pakistan J. Biol. Sci. 7(11): 1988-92. However, Aspergillus fumigatus is highly allergenic and pathogenic to humans and plants. Thus, Aspergillus fumigatus is not a viable production host for glucoamylases used in industrial processes for manufacturing products for human consumption. There is a need to produce A. fumigatus glucoamylases from a suitable host.